Solar cells which convert radiant energy from the sun into electrical energy are used to power spacecraft. Designers of power systems for this application face numerous design constraints of which weight is one of the most critical. The extraordinarily high cost of placing a pound of weight into space is a serious constraint, and any improvement which can reduce the weight and still provide adequate performance is actively sought.
Another constraint is the need for shielding the solar cells against radiation in regions of high fluence. This significantly increases the weight of the cells. Any arrangement which can reduce the area or volume that must be shielded is also actively sought.
Still another constraint is the high cost of the solar cells. These cells often are as much as 60% or more of the cost of the entire solar array. Reduction of required cell area, while still obtaining the same power output, is another sought-after advantage. Pursuit of this advantage leads to the consideration of Solar Concentrator Arrays. These arrays use optics to concentrate a given cross-sectional area of sunlight into a smaller solar cell area.
Yet another constraint is the volume required to pack a solar cell array into a launch vehicle for containment while being launched. Reduction of stowed envelope volume is another design objective. In most applications, the solar panels or sections are collapsed for storage and launch and require mechanisms to ensure proper deployment. Accordingly, simplicity of folding the array and storage during launch is an objective in this art. Additionally, a less complicated deployment mechanism removes possibilities of errors attributed to more complicated systems.
Accordingly, power generation efforts for spacecraft and other applications are driven by balancing a number of factors which include the following: cost, mass, reliability, complexity, and technical risk. For a concentrator array, key issues are: choice of concentration ratio; reflective versus refractive optics; cell choice and degradation analysis; array pointing accuracy (error tolerance); thermal control and operating temperature; and stowage and deployment approach and sequence.
Electrical power requirements for spacecraft applications are high. Current projections exist for systems up to 25 kW of power. Specific power generation capability, measured in watts of power per kilogram of power generator weight (W/kg), has been limited by the state of the art which, at this time is about 30-42 W/kg. Concentration ratio has been another limiting factor with 1.5:1 being relatively easy to achieve and 8:1 being the present state of the art.
Power generation degradation, i.e., reduced cell efficiency, is yet another constraint. With higher concentration of sun rays, higher temperatures are reached in the cells. Higher temperatures reduce the power conversion efficiency of the cells. Thus, there is a need to efficiently remove heat from the cells when higher concentration ratios are employed.
In the design of solar arrays, more favorable thermal environments are sought. The solar cells run more efficiently at cooler temperatures.